The present invention relates to a vulcanizable rubber composition containing an ethylene-propylene-nonconjugated diene copolymer rubber (EPDM) excellent in thermal resistance, fatigue resistance, and also in dynamic characteristics, low in tan xcex4 value and suitable for applications such as automobile tires and vibration-proof rubber. The present invention also relates to a vulcanized rubber prepared by vulcanizing the above-described rubber composition.
Diene rubbers such as natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR) are known as rubbers excellent in dynamic fatigue resistance and dynamic characteristics, and have been used as raw materials for automobile tires and vibration-proof rubber. However, the environment for the application of these rubber products has been largely changed recently to require improvements in thermal resistance and weather resistance of rubber products.
In automobile tires, tread and tire side walls especially require weather resistance. However, no rubber has been available which maintains the excellent fatigue resistance and dynamic characteristics of the existing diene rubber as well as having good weather resistance.
Thus various blended rubber compositions of diene rubber (which is excellent in dynamic fatigue resistance and dynamic characteristics) with EPT ((EPDM) which is excellent in thermal resistance and weather resistance) have been investigated. However, since the level of the dynamic characteristics of EPT is different from that of diene rubber, no blended rubber composition showing uniform properties could be prepared so far.
The dynamic characteristics of automobile tires are evaluated on the basis of whether the material increases fuel cost or not and its index tan xcex4 (loss tangent) value. The lower the tanxcex4 value, the better are the dynamic characteristics.
On the other hand, in the vibration-proof rubber products for automobiles, the existing vibration-proof rubber products based on natural rubber (which is a diene rubber) cannot provide sufficient fatigue resistance capable of withstanding practical use with the range of temperature increases in the engine compartment.
Therefore, there is a strong desire for a new rubber material having excellent thermal resistance, as well as dynamic characteristics and fatigue resistance, equal to or higher than those of diene rubber. Generally, in order to improve fatigue resistance, the rubber material is required to have a force relaxing mechanism. For this purpose, it is required to have the crosslinked form of the polysulfur bond in the rubber rather than a monosulfur bond. A proper crosslinking density is also required.
On the other hand, it is required to increase crosslinking density in order to improve dynamic characteristics. However, attempts to bring the dynamic characteristics of EPT in conformity with those of a diene rubber such as NR has resulted in too high a crosslinking density with the resultant deterioration of fatigue resistance; it was impossible to make dynamic characteristics compatible with fatigue resistance.
The dynamic characteristics in the vibration-proof rubber are evaluated on the basis of whether it has a low dynamic magnification. Since the dynamic magnification is approximately proportional to the tan xcex4 value, tan xcex4 can be used as its index.
The present invention discloses that the dynamic characteristics and the fatigue resistance, which are in opposite relation with respect to each other, can be unexpectedly improved at the same time by using (1) an ethylene-propylene-nonconjugated diene copolymer rubber (which is excellent in thermal resistance), (2) a specific alkoxysilane compound, and (3) a specific amorphous silica powder, to enhance the interaction between the amorphous silica powder and the polymer, that is, the ethylene-propylene-nonconjugated diene copolymer rubber through the alkoxysilane compound.
One object of the present invention is to solve the above-mentioned problems accompanying the conventional technology and to provide a rubber composition which has fatigue resistance and dynamic characteristics equal to those of a diene rubber (such as natural rubber) and which is also excellent in thermal resistance and weather resistance.
Another object of the present invention is to provide a vulcanized rubber comprising the above-mentioned rubber composition.
The vulcanizable rubber composition according to the present invention is characterized in that it contains an ethylene-propylene-nonconjugated diene copolymer rubber (EPDM), a crosslinking system as commonly used as well as at least one of the alkoxysilane compounds expressed by the following general formula (I) or (II), and silica and/or silicate powder having a specific surface area of 50 to 100 m2/g (BET adsorption: ISO 5794/1, Annex D).
The alkoxysilane has the general formula I: 
where R is an alkyl group having 1 to 4 carbon atoms or an alkoxy group having 1 to 4 carbon atoms; R1 is an alkyl group having 1 to 4 carbon atoms or phenyl groups; n is 0, 1 or 2; R2 is a divalent straight-chained or branched hydrocarbon radical (alkylene) having 1 to 6 carbon atoms; R3 is an arylene group having 6 to 12 carbon atoms; m and p are respectively 0 or 1 but not 0 at the same time; q is 1 or 2; and B is xe2x80x94SCN or xe2x80x94SH when q is 1 and xe2x80x94Sxxe2x80x94(where x is an integer of 2 to 8) when q is 2.
Alternatively, the alkoxysilane has the general formula 
where R is an alkyl group having 1 to 4 carbon atoms or an alkoxy group having 1 to 4 carbon atoms; R1 is an alkyl group having 1 to 4 carbon atoms or phenyl group; n is 0, 1 or 2; and R4 is a monovalent straight-chained or branched unsaturated hydrocarbon radical having 2 to 20 carbon atoms.
In the rubber composition according to the present invention, an ethylene-propylene-5-ethylidene-2-norbornene copolymer rubber having a Mooney viscosity (MS1+4, 160xc2x0 C.) of 40 to 80 is preferred as the above-mentioned ethylene-propylene-nonconjugated diene copolymer rubber.
The vulcanized rubber according to the present invention is characterized in that it is prepared by vulcanizing the rubber composition described above.
The rubber composition according to the present invention, and the vulcanized rubber prepared from the rubber composition, unexpectedly have fatigue resistances and dynamic characteristics equal to those of diene rubber (such as NR) and also are excellent in thermal resistances and weather resistances.
The rubber composition according to the present invention, and the vulcanized rubber prepared from the rubber composition, will be illustrated as follows:
First, the rubber composition according to the present invention will be described. The rubber composition according to the present invention contains (1) an ethylene-propylene-nonconjugated diene copolymer rubber, (2) a specific alkoxysilane compound, and (3) a specific amorphous silica and/or silicate powder.
The ethylene-propylene-nonconjugated diene copolymer rubber used in the present invention contains usually 50 to 83 mol % (preferably 50 to 73 mol %) of ethylene and usually 50 to 17 mol % (preferably 50 to 27 mol %) of propylene.
The above-mentioned nonconjugated dienes include, for example, (a) chain nonconjugated dienes such as 1,4-hexadiene; 1,6-octadiene; 2-methyl-1,5-hexadiene; 6-methyl-1,5-heptadiene; and 7-methyl-1,6-octadiene; (b) cyclic nonconjugated dienes such as cyclohexadiene; dicyclopentadiene; methyltetrahydroindene; 5-vinylnorbornene; 5-ethylidene-2-norbornene; 5-methylene-2-norbornene; 5-isopropylidene-2-norbornene; and 6-chlormethyl-5-isopropenyl-2-norbornene; and (c) trienes such as 2,3-diisopropylidene-5-norbornene; 2-ethylidene-3-isopropylidene-5-norbornene; 2-propenyl-2,2-norbornadiene; 1,3,7-octatriene; and 1,4,9-decatriene. Among them, preferably used are 1,4-hexadiene and cyclic nonconjugated dienes, particularly 5-ethylidene-2-norbornene. When 5-ethylidene-2-norbornene is used as the nonconjugated diene in the present invention, a rubber composition and a vulcanized rubber most excellent in fatigue resistance can be obtained.
The ethylene-propylene-nonconjugated diene copolymer rubber used in the present invention has a iodine number, an index for the nonconjugated diene content, of usually 8 to 30, preferably 8 to 25.
The ethylene-propylene-nonconjugated diene copolymer rubber used in the present invention has a Mooney viscosity (MS1+4, 160xc2x0 C.) of usually 40 to 80, preferably 50 to 80. When an ethylene-propylene-nonconjugated diene copolymer rubber having a Mooney viscosity (MS1+4, 160xc2x0 C.) falling within the above-mentioned range is used in the present invention, a rubber composition and a vulcanized rubber unexpectedly showing fatigue resistances equal to or higher than those of a diene rubber such as natural rubber can be prepared.
An ethylene-propylene-nonconjugated diene copolymer rubber having a Mooney viscosity (MS1+4, 100xc2x0 C.) of 60 to 200 can also be used in the claimed compositions.
Although the above-mentioned ethylene-propylene-nonconjugated diene copolymer rubber can be used alone as the rubber component, a blend of the above-mentioned copolymer rubber with a diene rubber can also be used. Such diene rubbers include, for example, natural rubber (NR), isopropylene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), and chloroprene rubber (CR). Among them, natural rubber and isoprene rubber are preferred. The above-mentioned diene rubbers are used either alone or in combination. The diene rubber is usually used in the present invention in an amount of 20 to 50 parts by weight based per 100 parts by weight of the total amount of the ethylene-propylene diene copolymer rubber.
The alkoxysilane compound used in the present invention is expressed by the general formula (I) or (II) and plays a part as a silane coupling agent. The alkoxysilane has the general formula I: 
where R is an alkyl group having 1 to 4 carbon atoms or an alkoxy group having 1 to 4 carbon atoms; R1 is an alkyl group having 1 to 4 carbon atoms or phenyl group; n is 0, 1 or 2; R2 is a divalent straight-chained or branched hydrocarbon radical having 1 to 6 carbon atoms; R3 is an arylene group having 6 to 12 carbon atoms; m and p are respectively 0 or 1 but not 0 at the same time; q is 1 or 2; and B is xe2x80x94SCN or xe2x80x94SH when q is 1 and xe2x80x94SXxe2x80x94 (where x is an integer of 2 to 8) when q is 2.
Alternatively, the alkoxysilane has the general formula II: 
where R, R1, and n have the same meanings as defined in the above general formula (I) and R4 is a monovalent straight chained or branched, unsaturated hydrocarbon radical having 2 to 20 carbon atoms.
Among the alkoxysilane compounds expressed by the above-mentioned general formula (I), trialkoxysilane compounds as shown below in which B in the general formula (I) is xe2x80x94S4xe2x80x94 are used preferably:
(1) Bis-3-(trimethoxysilyl)propyl-tetrasulfane
(H3CO)3 Sixe2x80x94(CH2)3xe2x80x94S4xe2x80x94(CH2)3xe2x80x94Sixe2x80x94(OCH3)3 
(2) Bis-3-(triethoxysilyl)propyl-tetrasulfane
(H5C2O)3Sixe2x80x94(CH2)3xe2x80x94Sxe2x80x94(CH2)3xe2x80x94Sixe2x80x94(OC2H5)3 
(3) Bis-3-(tripropoxysilyl)propyl-tetrasulfane
(H7C3 O )3Sixe2x80x94(CH2)3xe2x80x94S4xe2x80x94(CH2)3xe2x80x94Sixe2x80x94(OC3H7)3 
Among the above-mentioned compounds, particularly preferred is the above-mentioned (2), bis-3-(triethoxysilyl)propyl-tetrasulfane.
Among the alkoxysilane compounds expressed by the above-mentioned general formula (II), an alkoxysilane compound as shown below is preferably used:
(4) 3-Butene-triethoxysilane.
(C2H5 O)3Sixe2x80x94CH2CH2C=CH2 
A vulcanized rubber having excellent dynamic characteristics can be prepared by using the alkoxysilane compound expressed by the general formula (I) or (II) as shown above.
In the present invention, the alkoxysilane compound is used in an amount that ensures that 0.1xc2x710xe2x88x926 mol to 13.5xc2x710xe2x88x926 mol (preferably 0.3xc2x710xe2x88x926 to 10.5xc2x710xe2x88x926 mol) alkoxysilyl groups are available per square meter specific surface area of the amorphous silica. If the amount of alkoxysilyl groups becomes lower than 0.1xc3x9710xe2x88x926 mol per square meter surface area, the silica surface is modified to a too small extent giving to less filler/polymer crosslinks and too small effect on dynamic properties. If the amount of alkoxysilyl groups exceed 13.5xc2x710xe2x88x926 mol per square meter specific surface area, the silane is in excess compared to the reactive silica surface and has only little effect on further improvement of the dynamic compound properties.
A rubber composition excellent in thermal resistance, fatigue resistance, and dynamic characteristics can be obtained by using the alkoxysilane compound in the proportional amounts as shown above.
The amorphous silica powder used in the present invention are precipitated and hydrophilic fine powdered silicic acid or fine powdered silicates and have a specific surface area of 50 to 100 m2/g (BET adsorption: ISO 5794/1, Annex D) preferably 60 to 90 m2/g. In the present invention, the fine powdered silicic acid or the fine powdered silicates can be used either alone or in combination thereof.
In the present invention, the silica and/or silicates are used in a total amount of usually 5 to 90 parts by weight, preferably 20 to 80 parts by weight, based on 100 parts by weight of the rubber component.
When the rubber composition according to the present invention is used in a vibration-proof rubber product, there are required dynamic characteristics according to which vibration damping effect is exerted according to the application of the vibration-proof rubber product. Hence, the compounding proportions of the above-mentioned alkoxysilane compound and the amorphous silica powder are adjusted according to the purpose of the application.
In the present invention, additives such as inorganic fillers other than the above-mentioned amorphous silica and silicates can be incorporated into the rubber composition within the limit not impairing the purpose of the present invention.
The inorganic fillers other than the above-mentioned amorphous silica powder include, for example, carbon blacks such as SRF, GPF, FEF, HAF, ISAF, SAF, FT and MT, fine powdered silicic acid, light calcium carbonate, heavy calcium carbonate, talc and clay. In the rubber composition according to the present invention, the total amount of the inorganic filler components is usually 10 to 120 parts by weight based on 100 parts by weight of the rubber components. A too high total amount of the inorganic filler components cannot give a rubber composition and a vulcanized rubber excellent in dynamic characteristics and fatigue resistance.
Preparation of a vulcanized rubber from the rubber composition according to the present invention simply calls for preparation of unvulcanized compounded rubber (a rubber composition) once by the method mentioned below, and then molding of this compounded rubber to an intended shape, followed by vulcanization in the same manner as in vulcanizing a usual rubber. When the vulcanized rubber according to the present invention is prepared, the types and the amount added of the softening agent and also the types and the amount added of the compounds constituting the vulcanizing system (such as the vulcanizing agent, the vulcanization promotor, and the vulcanization aid), and the procedures for the preparation of the vulcanized rubber are properly selected, in addition to the above-mentioned rubber component, the alkoxysilane compound and the amorphous silica powder, in according with the intended application of the vulcanized rubber and the performance based on it.
A softening agent usually used in rubber can be used as the above-mentioned softening agent. Typically used are petroleum softening agents such as process oils, lubricating oils, paraffins, liquid paraffin, petroleum asphalt and vaseline; coal tar softeners such as coal tar and coal tar pitch; fatty oil softeners such as castor oil, linseed oil, rapeseed oil and coconut oil; tall oil or blends thereof in an amount of 0 to 60 parts by weight, preferably 2 to 40 parts by weight, based on 100 parts of rubber, as was well as waxes such as beeswax, carnauba wax and lanolin; fatty acids and fatty acid salts such as ricinolic acid, palmitic acid, barium stearate, calcium stearate and zinc laureate; and synthetic high polymers such as petroleum resin, atactic polypropylene and cumarone-indene resin. Among them, preferably used are the petroleum softeners, particularly process oils. Such agents and their amounts are well known in the art.
Sulfur compounds as shown below are used as the vulcanizing agents for the preparation of the vulcanized rubber according to the present invention. The sulfur compounds used include, for example, sulfur, sulfur chloride, sulfur dichloride, morpholine disulfide, an alkylphenol disulfide, tetramethyl-thiuram disulfide, and selenium dimethyldithio-carbamate. Among them, sulfur is preferably used. The above-mentioned sulfur compound is used in a proportion of 0.1 to 4 parts by weight, preferably 0.5 to 3 parts by weight, based on 100 parts by weight of the ethylene-propylene-nonconjugated diene copolymer rubber. All such substances are well known in the art.
It is preferred to use a vulcanization accelerator in combination when a sulfur compound is used as the vulcanizing agent in the preparation of a vulcanized rubber according to the present invention. Among the well known vulcanization accelerators useful for the invention are, for example, (a) thiazol compounds such as N-cyclohexyl-2-benzothiazol-sulfenamide; N-oxydiethylene-2-benzothiazol-sulfenamide; N,N-diisopropyl-2-benzothiazol-sulfenamide; 2-mercaptobenzothiazol; 2-(2,4-dinitrophenyl)mercaptobenzothiazol; 2-(2,6-diethyl-4-morpholinothio)benzothiazol and dibenzothiazyldisulfide; (b) guanidine compounds such as diphenylguanidine, triphenylguanidine, diorthotolyl-guanidine, orthotolyl biguanide and diphenylguanidine phthalate; (c) aldehyde-amine or aldehyde-ammonia compounds such as acetaldehyde-aniline reaction product, butyladehyde-aniline condensate, hexamethylene tetramine and acetaldehyde-ammonia reaction product; (d) imidazolin compounds such as 2-mercapto-imidazolin; (e) thiourea compounds such as thiocarbanilide, diethylthiourea, dibutylthiourea, trimethylthiourea and diorthotolylthiourea; (f) thiuram compounds such as tetramethylthiuram monosulfide, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutyl-thiuram disulfide and pentamethylene thiuram tetra-sulfide; (g) dithioate compounds such as zinc dimethyldithiocarbamate, zinc diethyldithiocarbamate, zinc di-n-butyldithiocarbamate, zinc ethylphenyl dithiocarbamate, zinc butylphenyl-dithiocarbamate, sodium dimethyldithiocarbamate, selenium dimethyldithiocarbamate and tellurium diethyldithiocarbamate; (h) xanthate compounds such as zinc dibutylxanthate; and (i) compounds such as zinc white.
The above-mentioned vulcanization accelerators are used in a proportion of 1 to 20 parts by weight, preferably 0.5 to 10 parts by weight, based on 100 parts by weight of the ethylene-propylene-nonconjugated diene copolymer rubber.
Alkoxysilanes and silicas or silicates, respectively, are preferably mixed or reacted prior to incorporation into the claimed rubber compounds, as described in U.S. Pat. No. 4,076,550 (incorporated by reference in its entirety) or German P 40 04 781 (U.S. Pat. No. 5,116,886, incorporated by reference in its entirety). It is not necessary that the total amount of silica or silicate used be modified with the alkoxy silanes. It is also possible to modify only part of it and to use the rest without preliminary modification.
Carbon blacks used can be pre-mixed or chemically modified with organosilicone compounds; their preparation is described in German Patent Application 40 23 537. The compounded rubber before vulcanization is prepared by the method shown below.
Thus, the above-mentioned rubber component, the alkoxysilane compound and the amorphous silica powder and further a softening agent are milled in a mixer such as a Banburry mixer at a temperature of 80 to 150xc2x0 C. for 3 to 10 minutes, and then a vulcanizing agent and, if required, a vulcanization accelerator or a vulcanization aid are added and mixed in a Banburry mixer or on a roll (such as an open roll) and milled at a roll temperature of 40 to 60xc2x0 C. for 5 to 30 minutes, and then the milled product is extruded to prepare compounded rubber in ribbon or sheet form. The compounded rubber thus prepared is molded to an intended shape by an extruder, a calendar roll or a press and heated at a temperature of 100 to 270xc2x0 C. for 1 to 150 minutes at the same time as the molding or after the molding is introduced in a vulcanizer to prepare vulcanized rubber. In performing such vulcanization, a mold may or may not be used. In case a mold is not used, the processes of molding and vulcanization are usually carried out continuously.
The rubber composition according to the present invention which consists of ethylene-propylene-nonconjugated diene copolymer rubber, an alkoxysilane compound, and amorphous silica powder is excellent not only in dynamic properties but also in mechanical properties, dynamic fatigue resistance and heat aging resistance, and can provide vulcanized rubber excellent in such properties. Since the vulcanized rubber obtained from the rubber composition according to the present invention is excellent in the above properties, it can be widely used as tires, automobile parts, general industrial parts, materials for civil engineering and building, and the like. In particular, it can be suitably used for uses in which dynamic fatigue resistance is required, e.g., tire treads, tire sidewalls, wiper blades, automobile engine mounts, etc.
The present inventon also concerns a rubber composition for engine mounts. Prior rubber compositions for engine mounts were made from natural rubber (NR) on account of its excellent elasticity with carbon black as reinforcing filler. Beside excellent elasticity, also required are excellent dynamic properties, low compression set and well balanced overall properties. Modern car design with improved aerodynamic properties leads to more encapsulated engines. Consequently, much higher temperatures occur in the engine area. High temperature leads to severe heat aging of NR-compounds and deterioration of the physical compound properties. Therefore the application of conventional NR engine mounts does not meet todays requirements. On account of the non-availability of alternatives, car producers are forced to use an unsatisfactory product.
In order to solve the problem of the car industry, the rubber composition of the present invention was developed which is heat stable and provides the necessary physical and dynamic properties. EPDM was selected due to it excellent heat aging resistance. However, EPDM has rather poor elasticity. Therefore, the conventional carbon black filler must be replaced by silica. But silica alone dose not impart the necessary performance. Only the combination with organosilances of the described types lead to the industry""s requested compound properties. The silica which is suitable for this application must be in the BET-surface area range of 50-100 m2/g. Silica with higher surface area exhibit less elasticity and higher compression set, while silica with lower surface area do not achieve the requested balance of overall properties. The rubber compositon of the present invention enables producers of engine mounts for cars to deliver products to the car industry which are unexpectedly superior to the conventional products. It enables the car industry to design cars with better aerodynamic properties, resulting in lower gasoline consumption which is also a benefit to the environment.